In traditional magnetic resonance imaging (MRI), a patient lies in a static magnetic field and is subjected to an imaging sequence using radiofrequency (RF) pulses and spatial and temporal magnetic field gradients. MRI uses the property of nuclear spin to collect image data. Nuclei with unpaired nucleons (protons or neutrons) possess a property known as spin, which results in a non-zero magnetic moment that can be used to conduct MRI, see, e.g., U.S. Pat. No. 5,397,987, which is incorporated by reference herein. Hydrogen nuclei have a single proton, and many MRI techniques utilize hydrogen nuclei since they are pervasive in human tissue. When a subject is placed in a main magnetic field, its nuclei align in the direction of the field (i.e., along the “magnetization axis”); the orientation of the nuclei can be represented by a magnetization vector, see, e.g., Horowitz, MRI Physics for Radiologists: A Visual Approach, 1995, which is incorporated by reference herein. In the classical physical description of magnetic resonance, these spinning nuclei can precess in a conical manner around the magnetization axis, generally out-of-phase with respect to each other.
To induce in-phase spinning at the resonance frequency of particular nuclei, a high-powered radio frequency excitation pulse, frequently in the kilowatt range, is broadcast at that resonance frequency. This RF pulse also causes the nuclei in a sample (e.g., a human brain) to rotate with respect to the magnetization vector created by the main magnetic field, see, e.g., Horowitz, MRI Physics for Radiologists: A Visual Approach, 1995, incorporated by reference herein. The spinning nuclei in the sample generate RF signals, which decay over time. Time-varying gradient magnetic fields are applied after the RF excitation pulse to permit spatial resolution of the decaying RF signals. Thus, the RF excitation pulse and the time-varying gradient magnetic fields together cause the sample to emit time-varying MR RF signals known as “free induction decay” (FID) signals. An antenna in the magnetic resonance (MR) scanner receives these FID signals, and these MR imaging signals are transmitted to a processor. The processor uses these signals to generate MR images that reflect the spatial distribution or chemical environment of the spinning nuclei.